1. Field of the Invention
The invention relates to a semiconductor wafer that can detect radiation, and more particularly, to a semiconductor wafer that uses a conducting state of a channel of a field oxide to measure radiance.
2. Description of the Prior Art
Due to the extensive use of semiconductor devices, semiconductor wafers are more often subjected to environments that have a large amount of radiation. This radiation can cause a semiconductor device to malfunction. Please refer to FIG. 1. FIG. 1 is a structural diagram of a prior art semiconductor wafer 10. The semiconductor wafer 10 has two N-type metal-oxide-semiconductor (NMOS) transistors 12A and 12B on a P-type substrate. Each NMOS transistor has a gate 16A or 16B, a source 18A or 18B, a drain 20A or 20B, and a gate oxide 22A or 22B. In the highly compact modern circuit layouts, distances between transistors are extremely small, and are isolated by field oxides, like field oxide 28 in the semiconductor wafer 10, to prevent mutual electrical interference. Conductive layer 26 on the field oxide 28 provides a link to each transistor. The conductive layer 26 is typically a metallic link in the semiconductor wafer 10. In addition, a channel stop 15 is below the field oxide 28.
When a semiconductor wafer 10 is subject to radiation, the energy of the radiation will create electron-hole pairs in the oxide layer in semiconductor wafer 10.Holes are more likely to be retained in the oxide layer because hole mobility is slower in an oxide layer. In a field oxide layer, the above phenomenon is more evident. Compared to other oxide layers (such as a gate oxide) in a semiconductor wafer, electron-hole pairs are more likely to occur in the field oxide layer, and holes are more likely to accumulate in a field oxide because the volume of a field oxide is larger.
When the conductive layer 26 passes over the field oxide 28, field oxide 28, conductive layer 26 and two electrodes 18A and 20B become, in effect, a field oxide transistor. Field oxide 28 is equivalent to a gate oxide capacitor. Charge carried by holes accumulated in the field oxide 28 reduces the threshold voltage of the equivalent field oxide transistor. It is well known that changing the amplitude of the threshold voltage of a metal-oxide-semiconductor is proportional to charge of the gate oxide capacitance, and inversely proportional to the capacitance of a gate oxide capacitor. In a metal-oxide-semiconductor transistor, when the gate oxide capacitance is very small, even very little net charge on the gate oxide capacitor will cause dramatic change of the threshold voltage. This change of threshold voltage in the above equivalent field oxide transistor is particularly evident. Because holes easily accumulate in the field oxide, and the field oxide is very thick, the equivalent gate capacitance of the field oxide is relatively small, and the threshold voltage caused by charge is thus affected more. If too much charge accumulates on the field oxide 28 because of radiation, the threshold voltage of the field oxide transistor is, in effect, reduced. If there is electric activity in the conductive layer 26, a channel will form below the field oxide 28 and activate the equivalent field oxide transistor. An improper electric connection between the electrode 18A and the electrode 20B on two sides of the field oxide 28 is formed. Then, the functionality of the field oxide 28 to isolate transistor 12A and transistor 12B is damaged and causes the semiconductor wafer 10 to malfunction.
In the prior art, the semiconductor wafer 10 has no advance warning that the semiconductor wafer 10 is being influenced by radiation. When negative affects induced by radiation accumulate to cause a prior art semiconductor wafer 10 to malfunction, normal operations of a microprocessor system based on the semiconductor wafer 10 is severely and adversely influenced.
It is therefore a primary objective of the present invention to provide a semiconductor wafer that can detect radiation and provide a warning signal when the semiconductor wafer is subjected to radiation-induced damage in the early stages of exposure.
Briefly, in a preferred embodiment, the present invention provides a semiconductor wafer having at least one sensor comprising. The sensor includes a field oxide transistor, and a detecting circuit electrically connected to the field oxide transistor for detecting if the field oxide transistor is switched on or off and generating corresponding detecting signals.
It is an advantage of the present invention that the semiconductor wafer according to the present invention can detect radiation and provide a corresponding warning. Malfunctions of a semiconductor wafer can thus be prevented.
These and other objects and the advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.